1. Field of the Invention
The present invention relates to an apparatus for protecting a motor from an overload (overcurrent), more particularly, to a method for setting and adjusting a trip sensitivity in a thermal overload protection apparatus.
2. Description of the Related Art
An overload protecting function, a basic function of a thermal overload trip apparatus, is implemented by performing a trip operation when an overload or overcurrent within a current range satisfying a pre-set condition for the trip operation is generated in an electric circuit. The current range may refer to a current range for the trip operation according to an IEC (International Electrotechnical Commission) standard specified as an international electrical standard. For example, a condition for the trip operation is that the trip operation should be performed within two hours when a current corresponding to 1.2 times a rated current is conducted in a circuit, and the trip operation should be performed for more than two hours and within several hours when a current corresponding to 1.05 times the rated current is conducted.
The thermal overload (overcurrent) trip apparatus generally includes a heater coil for generating heat when an overcurrent is generated, by being connected to the circuit and a bimetal winding about the heater coil so as to provide a driving force for a trip operation by being bent when the heater coil generates heat, as a driving actuator. One example of the thermal overload trip apparatus using the bimetal will be described with reference to FIGS. 1 and 2.
FIG. 1 is a diagram showing a configuration of a thermal overload trip apparatus in accordance with the related art, and FIG. 2 is a diagram showing a relation between an adjusting cam and a trip sensitivity adjusting range in the thermal overload trip apparatus in accordance with the related art.
In FIG. 1, a reference numeral 1 denotes bimetals. Here, three bimetals are provided so as to be connected to each circuit of three-phase AC. Thus, the bimetals are bent by heat from a heater coil (not shown) generating heat when an overcurrent is generated, and accordingly provide a driving force for a trip operation. A reference numeral 2 denotes a shifter mechanism. The shifter mechanism 2 transfers the driving force for the trip operation from the bimetals 1 and is movable in a horizontal direction in the plane of the drawing by contacting the bimetals 1 in right and left directions so as to receive the driving force provided from the bent bimetals 1. In FIG. 1, a reference numeral 3 denotes a trip mechanism. The trip mechanism 3 is biased to be rotated in a direction of the trip operation by a spring (reference numeral not given). In FIG. 1, a reference numeral 4 denotes a latch mechanism for releasing the trip mechanism 3 to be rotated in the direction of the trip operation or restricting the trip mechanism 3 not to be rotated in the direction of the trip operation. The latch mechanism 4 has one end portion facing a driving force transfer portion of the shifter mechanism 2 so as to receive the driving force from the shifter mechanism 2, another end portion disposed in a rotation path of the trip mechanism 3 so as to restrict or release the trip mechanism 3, and a middle portion therebetween supported by a rotation shaft (reference numeral not given) to be rotatable. A reference numeral 6 denotes a contact point between the trip mechanism 3 and the latch mechanism 4 at the restriction position. In FIG. 1, at a position contacting one portion of the latch mechanism 4, an adjusting knob mechanism 5 is disposed to be rotatable so as to displace the latch mechanism 4 to be closer to or to be distant from the shifter mechanism 2 resulting from variation of a contact pressure while contacting the latch mechanism 4. Here, the adjusting knob mechanism 5 includes a cam portion 9 having a radius varying according to a displacement angle of an outer circumference thereof, and an adjusting knob 10 coupled to the cam portion 9 or to integrally extended from the cam portion 9 so as to rotate the cam portion 9. In FIG. 1, a reference character y, as a bending displacement of the bimetals, indicates a predetermined displacement amount (distance) of the bending bimetals 1 when a predetermined overcurrent is conducted in the circuit. And, a reference numeral Δ y, as an allowance for trip operation, indicates a predetermined gap between the shifter mechanism 2 and the latch mechanism 4 when the shifter mechanism 2 is displaced by the pre-set bending amount y of the bimetals 1 caused by generation of the predetermined overcurrent. The allowance for trip operation is adjustable by the adjusting knob mechanism 5.
In the meantime, referring to FIG. 2, a configuration of the cam portion 9 included in the adjusting knob mechanism 5 in accordance with the related art will be described.
In FIG. 2, a reference character a indicates an adjustable cam range covering angles between a maximum trip operation insensitive adjusting position 12 and a maximum trip operation sensitive adjusting position 13. However, since a manufacturer of the thermal overload trip apparatus in the related art has adjusted an initial position of the cam portion 9 such as an initially-set position 11 for the cam portion 9 by rotating the adjusting knob 10 of FIG. 1 during manufacturing, a range allowing a user to substantially adjust the rotation angle of the cam portion 9 is a substantially-adjustable range b for the cam portion 9. In FIG. 2, a reference character c indicates an initially-set adjusting range for the cam.
Operation of the thermal overload trip apparatus in accordance with the related art will be described.
First, the trip operation will be described. When the heater coil (not shown) generates heat by the overcurrent on the circuit, the bimetals 1 are bent and moved rightward on the drawing. Accordingly, the shifter mechanism 2 is moved rightward in the plane of FIG. 1, that is in a shifter mechanism operating direction 7 applied when the overcurrent is generated by a value obtained by adding the allowance for trip operation Δ y to the bending amount y by the driving force of the bimetals 1 bent more than the value adding the allowance for trip operation Δ y to the bending amount y, accordingly the latch mechanism 4 is pressed rightward and then rotated in a counterclockwise direction in the plane of the drawing. Then, the trip mechanism 3 being restricted by the latch mechanism 4 is released and then rotated in the tripping direction, that is, in the counterclockwise direction by an elastic force of a spring (reference numeral not given), and accordingly a succeeding switching mechanism (not shown) is operated into a trip (circuit-opening) position and then the circuit is tripped (broken), thereby protecting the circuit and a load device.
Next, a sensitivity adjusting operation for the trip operation will be described with reference to FIGS. 1 and 2.
When the initial position of the cam portion 9 is adjusted such as the initially-set position 11 for the cam portion in FIG. 2, if the user rotates the cam portion 9 of FIG. 1 in the counterclockwise direction, the latch mechanism 4 is rotated in a clockwise direction centering the rotation shaft (reference numeral not given), that is, in a trip operation sensitivity adjusting direction 8, accordingly the allowance for trip operation Δ y becomes narrow and the trip operation sensitivity of the device with respect to the overcurrent increases.
In the above-mentioned thermal overcurrent trip apparatus according to the related art, the distance for adjusting a trip operation sensitivity of the device, that is, a bending amount y, is a very important factor for deciding whether or not the trip operation is implemented for an over load (overcurrent). And even though the trip operation is implemented by the cooperation between the trip load upon the trip apparatus and the elastic stress of the bimetal, an adjusting that reduces the remaining distance, which is the trip operation allowance Δy only to 0 (zero), has a drawback in that it is not capable of ensuring the reliability of a trip operation.
Moreover, adjusting the remaining distance between the shifter 2 and latch mechanism 4 (which is the trip operation allowance Δy, by an accurate distance, which is the accurate bending amount y that can be set,) works only if the manual rotating manipulation by a user is stopped at the exact instant when the trip apparatus operates to trip. However, the stop in the manual rotating manipulation has actually a very small velocity (not zero), so there is a drawback that a user manually rotating the knob cannot accurately adjust the sensitivity of the device.